1. Field of the Invention
The present invention relates to electronic musical instruments, and more particularly to the class of such instruments known as synthesizers.
2. The Prior Art
Electronic music synthesizers generally include an oscillator with means for selectively controlling the frequency produced by the oscillator, so that the output of the oscillator may be caused to produce musical tones and sounds. One component of a synthesizer is a tunable oscillator, and it is important that the oscillator remain in tune, without varying as a result of changes in temperature and other environmental conditions. If the oscillator does not inherently have the required stability, it must frequently be retuned, which is an inconvenience. In addition, rapid changes in tune (e.g., during warming up) are musically unsatisfactory.
In one class of synthesizers, a voltage divider is employed with several taps which are selected individually in accordance with the frequency of the signal which is desired to be produced by the oscillator. It is conventional to construct such a voltage divider by connecting in series several components which all have the same resistance, so that an equal voltage difference is developed by each change in the position of a selected tap, connected to the junction of adjacent components. It is necessary to use an oscillator arrangement which produces a frequency which is an exponential function of the control voltage, so that twelve successive taps produce the frequencies corresponding to the various notes of one octave of the musical scale.
Several designs for oscillators which have the required exponential function have been developed. In one such design, the oscillator is provided with a function generator for developing an exponential function in response to a linear input voltage, and a linear oscillator is controlled by the output of the function generator. This design has not been completely successful, because the function generator and the oscillator are both responsive to changes in the environmental condition, such as temperature, power supply voltage level, etc., and so the required stability has not been attained.
It is, therefore, desirable to produce a system in which the aforementioned disadvantages are overcome.